| 1 | // SPDX-License-Identifier: AGPL-3.0-or-later |
| 2 | |
| 3 | // Package sshkey wraps the parsing, validation, and fingerprinting of |
| 4 | // user-supplied SSH public keys. Every entry that takes key material |
| 5 | // from outside (the settings handlers, future imports, the AKC binary's |
| 6 | // equality checks) goes through this package so policy lives in exactly |
| 7 | // one place. |
| 8 | package sshkey |
| 9 | |
| 10 | import ( |
| 11 | "crypto/dsa" //nolint:staticcheck // DSA detection only — never accepted. |
| 12 | "crypto/ecdsa" |
| 13 | "crypto/ed25519" |
| 14 | "crypto/rsa" |
| 15 | "errors" |
| 16 | "fmt" |
| 17 | "strings" |
| 18 | |
| 19 | "golang.org/x/crypto/ssh" |
| 20 | ) |
| 21 | |
| 22 | // MinRSABits is the smallest accepted modulus length for ssh-rsa keys. |
| 23 | const MinRSABits = 2048 |
| 24 | |
| 25 | // MaxKeysPerUser is the cap enforced at the handler layer. Keeps DB rows |
| 26 | // per-user bounded and limits AKC lookup amplification if an attacker |
| 27 | // registered an enormous keyring on a single account. |
| 28 | const MaxKeysPerUser = 100 |
| 29 | |
| 30 | // Parsed is the validated, ready-to-store representation of a user-supplied |
| 31 | // SSH public key. Title is whatever name the user typed; everything else |
| 32 | // is derived from the key blob. |
| 33 | type Parsed struct { |
| 34 | Title string |
| 35 | Type string // OpenSSH algorithm name, e.g. "ssh-ed25519" |
| 36 | Bits int // 0 when not meaningful (e.g. ed25519) |
| 37 | Fingerprint string // canonical SHA256:<base64-no-padding> |
| 38 | // PublicKey is the canonical authorized_keys line (no comment; no |
| 39 | // trailing newline). It is what the AKC binary re-emits on a hit. |
| 40 | PublicKey string |
| 41 | } |
| 42 | |
| 43 | // Errors. Keep them precise enough for the settings handler to render a |
| 44 | // useful UI string. |
| 45 | var ( |
| 46 | ErrUnsupportedAlgo = errors.New("sshkey: unsupported key algorithm") |
| 47 | ErrRSATooShort = fmt.Errorf("sshkey: RSA keys must be at least %d bits", MinRSABits) |
| 48 | ErrUnparseable = errors.New("sshkey: could not parse key — paste the contents of your .pub file") |
| 49 | ErrTitleEmpty = errors.New("sshkey: title required") |
| 50 | ErrTitleTooLong = errors.New("sshkey: title may be at most 80 characters") |
| 51 | ErrTitleControl = errors.New("sshkey: title contains control characters") |
| 52 | ) |
| 53 | |
| 54 | // Parse validates and canonicalizes input. Title goes through a small |
| 55 | // validator (length + control-char rejection); the key blob through |
| 56 | // ssh.ParseAuthorizedKey + an algorithm whitelist + an RSA-bits check. |
| 57 | func Parse(title, blob string) (*Parsed, error) { |
| 58 | t := strings.TrimSpace(title) |
| 59 | if t == "" { |
| 60 | return nil, ErrTitleEmpty |
| 61 | } |
| 62 | if len(t) > 80 { |
| 63 | return nil, ErrTitleTooLong |
| 64 | } |
| 65 | if hasControlChars(t) { |
| 66 | return nil, ErrTitleControl |
| 67 | } |
| 68 | |
| 69 | pub, _, _, _, err := ssh.ParseAuthorizedKey([]byte(blob)) |
| 70 | if err != nil { |
| 71 | return nil, ErrUnparseable |
| 72 | } |
| 73 | |
| 74 | algo := pub.Type() |
| 75 | if !algorithmAllowed(algo) { |
| 76 | return nil, ErrUnsupportedAlgo |
| 77 | } |
| 78 | |
| 79 | bits, err := keyBits(pub) |
| 80 | if err != nil { |
| 81 | return nil, err |
| 82 | } |
| 83 | if algo == "ssh-rsa" && bits < MinRSABits { |
| 84 | return nil, ErrRSATooShort |
| 85 | } |
| 86 | |
| 87 | canonical := canonicalAuthorizedKey(pub) |
| 88 | return &Parsed{ |
| 89 | Title: t, |
| 90 | Type: algo, |
| 91 | Bits: bits, |
| 92 | Fingerprint: ssh.FingerprintSHA256(pub), |
| 93 | PublicKey: canonical, |
| 94 | }, nil |
| 95 | } |
| 96 | |
| 97 | // FingerprintOf returns the canonical SHA256 fingerprint for an |
| 98 | // already-parsed key. Used by the AKC binary's equality checks. |
| 99 | func FingerprintOf(pub ssh.PublicKey) string { |
| 100 | return ssh.FingerprintSHA256(pub) |
| 101 | } |
| 102 | |
| 103 | // canonicalAuthorizedKey re-emits the parsed key as a single |
| 104 | // `<algo> <base64>` line so storage/lookup don't accidentally depend on |
| 105 | // whitespace or comment fields the user supplied. |
| 106 | func canonicalAuthorizedKey(pub ssh.PublicKey) string { |
| 107 | line := strings.TrimSpace(string(ssh.MarshalAuthorizedKey(pub))) |
| 108 | return line |
| 109 | } |
| 110 | |
| 111 | // algorithmAllowed gates the OpenSSH algorithm names we accept. |
| 112 | func algorithmAllowed(algo string) bool { |
| 113 | switch algo { |
| 114 | case "ssh-ed25519", |
| 115 | "sk-ssh-ed25519@openssh.com", |
| 116 | "ecdsa-sha2-nistp256", |
| 117 | "ecdsa-sha2-nistp384", |
| 118 | "ecdsa-sha2-nistp521", |
| 119 | "sk-ecdsa-sha2-nistp256@openssh.com", |
| 120 | "ssh-rsa": |
| 121 | return true |
| 122 | } |
| 123 | return false |
| 124 | } |
| 125 | |
| 126 | // keyBits extracts the modulus / curve size from a parsed key. Returns 0 |
| 127 | // for ed25519 (where the concept doesn't apply meaningfully). |
| 128 | func keyBits(pub ssh.PublicKey) (int, error) { |
| 129 | cp, ok := pub.(ssh.CryptoPublicKey) |
| 130 | if !ok { |
| 131 | return 0, nil |
| 132 | } |
| 133 | switch k := cp.CryptoPublicKey().(type) { |
| 134 | case ed25519.PublicKey: |
| 135 | return 0, nil |
| 136 | case *rsa.PublicKey: |
| 137 | return k.N.BitLen(), nil |
| 138 | case *ecdsa.PublicKey: |
| 139 | return k.Params().BitSize, nil |
| 140 | case *dsa.PublicKey: |
| 141 | return 0, ErrUnsupportedAlgo |
| 142 | default: |
| 143 | return 0, nil |
| 144 | } |
| 145 | } |
| 146 | |
| 147 | func hasControlChars(s string) bool { |
| 148 | for _, r := range s { |
| 149 | if r < 0x20 || r == 0x7f { |
| 150 | return true |
| 151 | } |
| 152 | } |
| 153 | return false |
| 154 | } |
| 155 |